Understanding the impact of surface roughness: changing from FTO to ITO to PEN/ITO for flexible perovskite solar cells

So far, single-junction flexible PSCs have been lacking in efficiency compared to rigid PSCs. Recently, > 23% have been reported. We therefore focus on understanding the differences between rigid and flexible substrates. One often neglected parameter is the different surface roughness which directly affects the perovskite film formation. Therefore, we adjust the layer thickness of SnO2 and the perovskite layers. Furthermore, we introduce a PMMA layer between the perovskite and the hole transporting material (HTM), spiro-MeOTAD, to mitigate shunting pathways. In addition, the multication perovskite Rb0.02Cs0.05FA0.77MA0.16Pb(I0.83Br0.17)3 is employed, resulting in stabilized performances of 16% for a flexible ITO substrate and 19% on a rigid ITO substrate.

www.nature.com/scientificreports/ We identify three areas precluding rigid PSCs with FTO from the transfer to flexible ITO substrates. First, the electron transport material layer, SnO 2 , requires a different layer thickness. Secondly, the perovskite layer thickness itself needs changing and thirdly a thin interfacial PMMA layer is introduced to prevent shunting pathways between the HTM, spiro-OMeTAD, and the ETM, SnO 2 . Following these steps, rigid ITO devices were achieved with a stabilized PCE of 19.1% and flexible substrates with a stabilized PCE of 16%. We posit that one of the main challenges for achieving highly-efficient flexible perovskite solar is the different surface roughness.

Towards efficient, flexible devices-the differences between ITO and FTO as transparent conductive oxides for perovskite solar cells
Most rigid PSCs have been processed on FTO, which is unsuitable for flexible PSCs as it requires a high processing temperature when manufactured. It is, therefore, necessary to change from FTO to ITO. The two materials have different Fermi levels due to their different dopants, influencing the energy band level alignments to the other materials of the solar cell stack. For this work, however, the highest open-circuit voltage (V oc ) is at 1.18 V for ITO, which is comparable to some of the highest reported V oc for FTO and SnO 2 1.21 V 26 . The energy band alignment levels depend on several factors, such as the work function, the substrate treatment, deposition method, and interfaces, involving the possible occurrence of interface dipoles 28,29 . The discussion about energy band alignments in perovskites often does not consider the critical relationship between surface roughness and subsequently altered film formation. Following this, we further characterized the surfaces in terms of roughness and contact angle (see Table 1). Our hypothesis is that the smoother surface of ITO influence the perovskite film formation significantly, leading to more regular, block-like grain boundaries.
The measured contact angle for a polished FTO substrate is 58.0° ± 1.19°, which lies between the surface roughness of FTO and ITO. This is consistent with the trend of better wettability, correlating with increasing surface roughness for contact angle below 90°3 0 . Cross-section and top-view scanning electron microscopy (SEM) images of ITO resp. FTO with a 15 nm SnO 2 layer and a perovskite layer are shown in Fig. 1. The rough FTO substrate has a perovskite layer with numerous, ragged grain boundaries, and other irregularities, seeming to have a less ordered orientation. In contrast, the crystals on the smooth ITO substrate are more distinct and block-like. The grain boundaries tend to go from the bottom to the top contact. We checked our hypothesis on a polished FTO substrate, which has less surface roughness. In Fig. S1, we observed more similar grain behavior in line with the smooth ITO surface. Thus, there is a trend that a smoother surface correlates with more regular, block-like grain boundaries. www.nature.com/scientificreports/ Although all fabrication parameters and the precursor solution were the same for all devices, the perovskite layer thickness on the ITO is only 340 nm which is about 2/3 of the layer thickness on FTO. We measured the contact angle of the perovskite precursor on ITO, polished FTO and FTO (all with a SnO 2 compact layer), showing that the wettability correlates with surface roughness (see Table 2), which resulted in a thinner perovskite thickness. These thinner layers harvest less incoming light, and consequently, full devices on ITO showed a lower current density of 19-20 mA cm −2 compared to FTO with around 22 mA cm −2 . Furthermore, the ratio of shunted solar cells occurred more frequently for smooth ITO than for rough FTO, with a reduced fill factor (FF) of 10% for ITO compared to FTO. These observations can be linked to the SEM images ( Fig. 1): ITO shows a higher abundance of cracks due to the more monolithic crystal growth that also may lead to a thinner perovskite layer (Fig. 1A,C) compared to the ones on FTO (Fig. 1B,D). Lowering the shunt resistance resulting in a decreased fill factor which is more likely to occur on smooth ITO. Based on these observations, we use three optimization steps to tackle the different substrate properties to reach higher efficient devices: Firstly, the smoother surface of the ITO allows for a thinner SnO 2 layer which improves the charge carrier extraction as shown by Stolterfoht et al. for PTAA 31 . In general, considerably thicker layers are required to ensure a pinhole-free layer due to the roughness of the transparent conductive oxide (TCO). With a smoother surface, the layer thickness can be reduced without risking pinholes that increase the risk of shunting. Samples with a SnO 2 thickness of 2 to 10 nm and with 15 nm (the current standard for the FTO reference) were fabricated. As shown in Fig. 2A, a slight performance improvement could be achieved down to 5 nm without reducing reproducibility. Below 5 nm more devices get shunted as the SnO 2 layer does not fully cover the ITO electrode anymore. Since the reproducibility is higher, and the average efficiency is not significantly lower, we decided to use 5 nm as the optimal thickness for the planar architecture on ITO. Lower thickness can achieve higher efficiencies but also reduces reproducibility. We hypothesize, as observed by Stolterfoht et al. 31 , that the thinner charge transport layer leads to a faster charge extraction and thus to a lower recombination rate with higher performance.
The second optimization targets the decreased current density observed for the different wettability of the ITO substrate compared to the FTO substrates. We observed no difference between the flexible and rigid substrates. To increase the short circuit current density (J SC ), we increase the layer thickness by lowering the maximum spin speed during the perovskite deposition. The correlation of the layer thickness as a function of spin speed is shown in Fig. S2. The J-V curves show an improved current density by ca. 1-2 mA cm −2 without decreasing the reproducibility.
The third optimization approach targets the voids between the perovskite crystals and the consequent shunting. It is likely that the conductive spiro-OMeTAD penetrates through the voids to directly contact the ETL. Therefore, a thin isolating buffer layer between the perovskite layer and the spiro-OMeTAD could prevent such a shunting pathway. Thus, a thin layer of Poly(methyl methacrylate) PMMA, using a 0.1 mg/ml solution in chlorobenzene (CB), is deposited on top of the perovskite layer before the HTM. The introduction of an additional PMMA layer reduces the fraction of shunted devices from about 80% to 0%. We hypothesize that since we spin-coat spiro-OMeTAD, dissolved in CB, on top of the PMMA layer, the HTM solution may dissolve the PMMA on the surface but not the PMMA penetrated within the voids in the perovskite layer. This seems likely as we observe no difference in the J SC (20.3 ± 0.6 mA cm −2 for the control, compared to 20.5 ± 0.3 mA cm −2 for the PMMA devices), V OC (1.13 ± 0.02 V for the control, compared to 1.13 ± 0.02 V for the PMMA devices), and FF (0.68 ± 0.04 compared to 0.68 ± 0.05).
Finally, we use an architecture with 5 nm of SnO 2 , 480 nm of RbCsMAFA perovskite with PMMA on top, followed by spiro-OMeTAD. All three optimizations (SnO 2 and perovskite layer thickness adjustment, additional PMMA layer) together lead to a PSC on a rigid ITO substrate with a stabilized PCE of 19.1% as shown in Fig. 2B. We used the same procedure to transfer this architecture onto flexible substrates with PET/ITO. Without any further modification, it was possible to use the previously used optimization for rigid ITO. We achieved up to 16% stabilized power output as shown in Fig. 2C. The efficiencies of 15 devices on rigid and flexible ITO substrates are shown in Fig. 2D (all parameters are shown in Fig. S3).
Our fully optimized rigid PSCs show efficiency improvements to the original parameters with FTO from 6 to 7% (absolute), from 12% to a stabilized PCE of 19.1%. The most significant improvement however, is the increase in the reproducibility of the cells. The fraction of shunted devices decreased drastically from about 80% to 0% in the fully optimized architecture. Considering that ITO has a different work function than FTO, the V OC remains high at 1.18 V ( Table 2). The parameter which stays relatively low and hampers the cell from going towards 20% efficiency and beyond is the J SC . The highest J SC achieved so far in this work was only 21.17 mA cm −2 . Planar cells on FTO reached a J SC close to 26 mA cm −232 . The PCE of the solar cells fabricated on the flexible ITO substrate reached efficiencies of 16%, which is 3.1% less efficient than on the ITO substrate. The reduction in efficiency originates from the low fill factor of 0.69 and the lower J SC of 19.7 mA cm −2 . The variances might stem from further differences between the rigid and flexible ITO substrates, like macroscopic substrate planarity.

Conclusion
We provide guidelines on transferring from conventional but rough FTO, which is not compatible with a flexible substrate, to smooth ITO. Surface roughness emerges as highly important, affecting the perovskite growth drastically, with the smooth substrate (ITO) exhibiting more monolithic film formation. Following this, three optimization approaches were implemented, such as a reduction of the SnO 2 layer thickness from 15 to 5 nm, an increase in perovskite film thickness through a lowered spin speed, and a thin PMMA buffer layer. This results in PSCs with a stabilized efficiency of 19.1% on a rigid ITO substrate and stabilized 16% on a flexible ITO/PET substrate. This optimization work on ITO is an important step towards the commercialization of flexible PSCs and gives general guidelines on transferring optimized perovskite architecture from FTO to ITO.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author.